/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.locks;

import java.util.*;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.*;
import sun.misc.Unsafe;
import vjava.util.concurrent.*;

/**
 * Provides a framework for implementing blocking locks and related
 * synchronizers (semaphores, events, etc) that rely on first-in-first-out
 * (FIFO) wait queues. This class is designed to be a useful basis for most
 * kinds of synchronizers that rely on a single atomic <tt>int</tt> value to
 * represent state. Subclasses must define the protected methods that change
 * this state, and which define what that state means in terms of this object
 * being acquired or released. Given these, the other methods in this class
 * carry out all queuing and blocking mechanics. Subclasses can maintain other
 * state fields, but only the atomically updated <tt>int</tt> value manipulated
 * using methods {@link #getState}, {@link #setState} and
 * {@link #compareAndSetState} is tracked with respect to synchronization.
 * 
 * <p>
 * Subclasses should be defined as non-public internal helper classes that are
 * used to implement the synchronization properties of their enclosing class.
 * Class <tt>AbstractQueuedSynchronizer</tt> does not implement any
 * synchronization interface. Instead it defines methods such as
 * {@link #acquireInterruptibly} that can be invoked as appropriate by concrete
 * locks and related synchronizers to implement their public methods.
 * 
 * <p>
 * This class supports either or both a default <em>exclusive</em> mode and a
 * <em>shared</em> mode. When acquired in exclusive mode, attempted acquires by
 * other threads cannot succeed. Shared mode acquires by multiple threads may
 * (but need not) succeed. This class does not &quot;understand&quot; these
 * differences except in the mechanical sense that when a shared mode acquire
 * succeeds, the next waiting thread (if one exists) must also determine whether
 * it can acquire as well. Threads waiting in the different modes share the same
 * FIFO queue. Usually, implementation subclasses support only one of these
 * modes, but both can come into play for example in a {@link ReadWriteLock}.
 * Subclasses that support only exclusive or only shared modes need not define
 * the methods supporting the unused mode.
 * 
 * <p>
 * This class defines a nested {@link ConditionObject} class that can be used as
 * a {@link Condition} implementation by subclasses supporting exclusive mode
 * for which method {@link #isHeldExclusively} reports whether synchronization
 * is exclusively held with respect to the current thread, method
 * {@link #release} invoked with the current {@link #getState} value fully
 * releases this object, and {@link #acquire}, given this saved state value,
 * eventually restores this object to its previous acquired state. No
 * <tt>AbstractQueuedSynchronizer</tt> method otherwise creates such a
 * condition, so if this constraint cannot be met, do not use it. The behavior
 * of {@link ConditionObject} depends of course on the semantics of its
 * synchronizer implementation.
 * 
 * <p>
 * This class provides inspection, instrumentation, and monitoring methods for
 * the internal queue, as well as similar methods for condition objects. These
 * can be exported as desired into classes using an
 * <tt>AbstractQueuedSynchronizer</tt> for their synchronization mechanics.
 * 
 * <p>
 * Serialization of this class stores only the underlying atomic integer
 * maintaining state, so deserialized objects have empty thread queues. Typical
 * subclasses requiring serializability will define a <tt>readObject</tt> method
 * that restores this to a known initial state upon deserialization.
 * 
 * <h3>Usage</h3>
 * 
 * <p>
 * To use this class as the basis of a synchronizer, redefine the following
 * methods, as applicable, by inspecting and/or modifying the synchronization
 * state using {@link #getState}, {@link #setState} and/or
 * {@link #compareAndSetState}:
 * 
 * <ul>
 * <li> {@link #tryAcquire}
 * <li> {@link #tryRelease}
 * <li> {@link #tryAcquireShared}
 * <li> {@link #tryReleaseShared}
 * <li> {@link #isHeldExclusively}
 * </ul>
 * 
 * Each of these methods by default throws {@link UnsupportedOperationException}
 * . Implementations of these methods must be internally thread-safe, and should
 * in general be short and not block. Defining these methods is the
 * <em>only</em> supported means of using this class. All other methods are
 * declared <tt>final</tt> because they cannot be independently varied.
 * 
 * <p>
 * You may also find the inherited methods from
 * {@link AbstractOwnableSynchronizer} useful to keep track of the thread owning
 * an exclusive synchronizer. You are encouraged to use them -- this enables
 * monitoring and diagnostic tools to assist users in determining which threads
 * hold locks.
 * 
 * <p>
 * Even though this class is based on an internal FIFO queue, it does not
 * automatically enforce FIFO acquisition policies. The core of exclusive
 * synchronization takes the form:
 * 
 * <pre>
 * Acquire:
 *     while (!tryAcquire(arg)) {
 *        <em>enqueue thread if it is not already queued</em>;
 *        <em>possibly block current thread</em>;
 *     }
 * 
 * Release:
 *     if (tryRelease(arg))
 *        <em>unblock the first queued thread</em>;
 * </pre>
 * 
 * (Shared mode is similar but may involve cascading signals.)
 * 
 * <p>
 * <a name="barging">Because checks in acquire are invoked before enqueuing, a
 * newly acquiring thread may <em>barge</em> ahead of others that are blocked
 * and queued. However, you can, if desired, define <tt>tryAcquire</tt> and/or
 * <tt>tryAcquireShared</tt> to disable barging by internally invoking one or
 * more of the inspection methods, thereby providing a <em>fair</em> FIFO
 * acquisition order. In particular, most fair synchronizers can define
 * <tt>tryAcquire</tt> to return <tt>false</tt> if
 * {@link #hasQueuedPredecessors} (a method specifically designed to be used by
 * fair synchronizers) returns <tt>true</tt>. Other variations are possible.
 * 
 * <p>
 * Throughput and scalability are generally highest for the default barging
 * (also known as <em>greedy</em>, <em>renouncement</em>, and
 * <em>convoy-avoidance</em>) strategy. While this is not guaranteed to be fair
 * or starvation-free, earlier queued threads are allowed to recontend before
 * later queued threads, and each recontention has an unbiased chance to succeed
 * against incoming threads. Also, while acquires do not &quot;spin&quot; in the
 * usual sense, they may perform multiple invocations of <tt>tryAcquire</tt>
 * interspersed with other computations before blocking. This gives most of the
 * benefits of spins when exclusive synchronization is only briefly held,
 * without most of the liabilities when it isn't. If so desired, you can augment
 * this by preceding calls to acquire methods with "fast-path" checks, possibly
 * prechecking {@link #hasContended} and/or {@link #hasQueuedThreads} to only do
 * so if the synchronizer is likely not to be contended.
 * 
 * <p>
 * This class provides an efficient and scalable basis for synchronization in
 * part by specializing its range of use to synchronizers that can rely on
 * <tt>int</tt> state, acquire, and release parameters, and an internal FIFO
 * wait queue. When this does not suffice, you can build synchronizers from a
 * lower level using {@link vjava.util.concurrent.atomic atomic} classes, your
 * own custom {@link java.util.Queue} classes, and {@link LockSupport} blocking
 * support.
 * 
 * <h3>Usage Examples</h3>
 * 
 * <p>
 * Here is a non-reentrant mutual exclusion lock class that uses the value zero
 * to represent the unlocked state, and one to represent the locked state. While
 * a non-reentrant lock does not strictly require recording of the current owner
 * thread, this class does so anyway to make usage easier to monitor. It also
 * supports conditions and exposes one of the instrumentation methods:
 * 
 * <pre>
 * class Mutex implements Lock, java.io.Serializable {
 * 
 * 	// Our internal helper class
 * 	private static class Sync extends AbstractQueuedSynchronizer {
 * 		// Report whether in locked state
 * 		protected boolean isHeldExclusively() {
 * 			return getState() == 1;
 * 		}
 * 
 * 		// Acquire the lock if state is zero
 * 		public boolean tryAcquire(int acquires) {
 * 			assert acquires == 1; // Otherwise unused
 * 			if (compareAndSetState(0, 1)) {
 * 				setExclusiveOwnerThread(Thread.currentThread());
 * 				return true;
 * 			}
 * 			return false;
 * 		}
 * 
 * 		// Release the lock by setting state to zero
 * 		protected boolean tryRelease(int releases) {
 * 			assert releases == 1; // Otherwise unused
 * 			if (getState() == 0)
 * 				throw new IllegalMonitorStateException();
 * 			setExclusiveOwnerThread(null);
 * 			setState(0);
 * 			return true;
 * 		}
 * 
 * 		// Provide a Condition
 * 		Condition newCondition() {
 * 			return new ConditionObject();
 * 		}
 * 
 * 		// Deserialize properly
 * 		private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
 * 			s.defaultReadObject();
 * 			setState(0); // reset to unlocked state
 * 		}
 * 	}
 * 
 * 	// The sync object does all the hard work. We just forward to it.
 * 	private final Sync sync = new Sync();
 * 
 * 	public void lock() {
 * 		sync.acquire(1);
 * 	}
 * 
 * 	public boolean tryLock() {
 * 		return sync.tryAcquire(1);
 * 	}
 * 
 * 	public void unlock() {
 * 		sync.release(1);
 * 	}
 * 
 * 	public Condition newCondition() {
 * 		return sync.newCondition();
 * 	}
 * 
 * 	public boolean isLocked() {
 * 		return sync.isHeldExclusively();
 * 	}
 * 
 * 	public boolean hasQueuedThreads() {
 * 		return sync.hasQueuedThreads();
 * 	}
 * 
 * 	public void lockInterruptibly() throws InterruptedException {
 * 		sync.acquireInterruptibly(1);
 * 	}
 * 
 * 	public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
 * 		return sync.tryAcquireNanos(1, unit.toNanos(timeout));
 * 	}
 * }
 * </pre>
 * 
 * <p>
 * Here is a latch class that is like a {@link CountDownLatch} except that it
 * only requires a single <tt>signal</tt> to fire. Because a latch is
 * non-exclusive, it uses the <tt>shared</tt> acquire and release methods.
 * 
 * <pre>
 * class BooleanLatch {
 * 
 * 	private static class Sync extends AbstractQueuedSynchronizer {
 * 		boolean isSignalled() {
 * 			return getState() != 0;
 * 		}
 * 
 * 		protected int tryAcquireShared(int ignore) {
 * 			return isSignalled() ? 1 : -1;
 * 		}
 * 
 * 		protected boolean tryReleaseShared(int ignore) {
 * 			setState(1);
 * 			return true;
 * 		}
 * 	}
 * 
 * 	private final Sync sync = new Sync();
 * 
 * 	public boolean isSignalled() {
 * 		return sync.isSignalled();
 * 	}
 * 
 * 	public void signal() {
 * 		sync.releaseShared(1);
 * 	}
 * 
 * 	public void await() throws InterruptedException {
 * 		sync.acquireSharedInterruptibly(1);
 * 	}
 * }
 * </pre>
 * 
 * @since 1.5
 * @author Doug Lea
 */
public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer implements java.io.Serializable {

	private static final long serialVersionUID = 7373984972572414691L;

	/**
	 * Creates a new <tt>AbstractQueuedSynchronizer</tt> instance with initial
	 * synchronization state of zero.
	 */
	protected AbstractQueuedSynchronizer() {
	}

	/**
	 * Wait queue node class.
	 * 
	 * <p>
	 * The wait queue is a variant of a "CLH" (Craig, Landin, and Hagersten)
	 * lock queue. CLH locks are normally used for spinlocks. We instead use
	 * them for blocking synchronizers, but use the same basic tactic of holding
	 * some of the control information about a thread in the predecessor of its
	 * node. A "status" field in each node keeps track of whether a thread
	 * should block. A node is signalled when its predecessor releases. Each
	 * node of the queue otherwise serves as a specific-notification-style
	 * monitor holding a single waiting thread. The status field does NOT
	 * control whether threads are granted locks etc though. A thread may try to
	 * acquire if it is first in the queue. But being first does not guarantee
	 * success; it only gives the right to contend. So the currently released
	 * contender thread may need to rewait.
	 * 
	 * <p>
	 * To enqueue into a CLH lock, you atomically splice it in as new tail. To
	 * dequeue, you just set the head field.
	 * 
	 * <pre>
	 *      +------+  prev +-----+       +-----+
	 * head |      | <---- |     | <---- |     |  tail
	 *      +------+       +-----+       +-----+
	 * </pre>
	 * 
	 * <p>
	 * Insertion into a CLH queue requires only a single atomic operation on
	 * "tail", so there is a simple atomic point of demarcation from unqueued to
	 * queued. Similarly, dequeing involves only updating the "head". However,
	 * it takes a bit more work for nodes to determine who their successors are,
	 * in part to deal with possible cancellation due to timeouts and
	 * interrupts.
	 * 
	 * <p>
	 * The "prev" links (not used in original CLH locks), are mainly needed to
	 * handle cancellation. If a node is cancelled, its successor is (normally)
	 * relinked to a non-cancelled predecessor. For explanation of similar
	 * mechanics in the case of spin locks, see the papers by Scott and Scherer
	 * at http://www.cs.rochester.edu/u/scott/synchronization/
	 * 
	 * <p>
	 * We also use "next" links to implement blocking mechanics. The thread id
	 * for each node is kept in its own node, so a predecessor signals the next
	 * node to wake up by traversing next link to determine which thread it is.
	 * Determination of successor must avoid races with newly queued nodes to
	 * set the "next" fields of their predecessors. This is solved when
	 * necessary by checking backwards from the atomically updated "tail" when a
	 * node's successor appears to be null. (Or, said differently, the
	 * next-links are an optimization so that we don't usually need a backward
	 * scan.)
	 * 
	 * <p>
	 * Cancellation introduces some conservatism to the basic algorithms. Since
	 * we must poll for cancellation of other nodes, we can miss noticing
	 * whether a cancelled node is ahead or behind us. This is dealt with by
	 * always unparking successors upon cancellation, allowing them to stabilize
	 * on a new predecessor, unless we can identify an uncancelled predecessor
	 * who will carry this responsibility.
	 * 
	 * <p>
	 * CLH queues need a dummy header node to get started. But we don't create
	 * them on construction, because it would be wasted effort if there is never
	 * contention. Instead, the node is constructed and head and tail pointers
	 * are set upon first contention.
	 * 
	 * <p>
	 * Threads waiting on Conditions use the same nodes, but use an additional
	 * link. Conditions only need to link nodes in simple (non-concurrent)
	 * linked queues because they are only accessed when exclusively held. Upon
	 * await, a node is inserted into a condition queue. Upon signal, the node
	 * is transferred to the main queue. A special value of status field is used
	 * to mark which queue a node is on.
	 * 
	 * <p>
	 * Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill Scherer and
	 * Michael Scott, along with members of JSR-166 expert group, for helpful
	 * ideas, discussions, and critiques on the design of this class.
	 */
	static final class Node {
		/** Marker to indicate a node is waiting in shared mode 标记是共享模式 */
		static final Node SHARED = new Node();
		/** Marker to indicate a node is waiting in exclusive mode 标记是独占模式 */
		static final Node EXCLUSIVE = null;

		/**
		 * waitStatus value to indicate thread has cancelled 代表线程已经被取消
		 */
		static final int CANCELLED = 1;

		/**
		 * waitStatus value to indicate successor's thread needs unparking
		 * 代表后续节点需要唤醒
		 */
		static final int SIGNAL = -1;

		/**
		 * waitStatus value to indicate thread is waiting on condition
		 * 代表线程在condition queue中，等待某一条件
		 */
		static final int CONDITION = -2;

		/**
		 * waitStatus value to indicate the next acquireShared should
		 * unconditionally propagate
		 * 代表后续结点会传播唤醒的操作
		 */
		static final int PROPAGATE = -3;

		/**
		 * Status field, taking on only the values: SIGNAL: The successor of
		 * this node is (or will soon be) blocked (via park), so the current
		 * node must unpark its successor when it releases or cancels. To avoid
		 * races, acquire methods must first indicate they need a signal, then
		 * retry the atomic acquire, and then, on failure, block. CANCELLED:
		 * This node is cancelled due to timeout or interrupt. Nodes never leave
		 * this state. In particular, a thread with cancelled node never again
		 * blocks. CONDITION: This node is currently on a condition queue. It
		 * will not be used as a sync queue node until transferred, at which
		 * time the status will be set to 0. (Use of this value here has nothing
		 * to do with the other uses of the field, but simplifies mechanics.)
		 * PROPAGATE: A releaseShared should be propagated to other nodes. This
		 * is set (for head node only) in doReleaseShared to ensure propagation
		 * continues, even if other operations have since intervened. 0: None of
		 * the above
		 * 
		 * The values are arranged numerically to simplify use. Non-negative
		 * values mean that a node doesn't need to signal. So, most code doesn't
		 * need to check for particular values, just for sign.
		 * 
		 * The field is initialized to 0 for normal sync nodes, and CONDITION
		 * for condition nodes. It is modified using CAS (or when possible,
		 * unconditional volatile writes).
		 * <p>
		 * 结点的等待状态。
		 * <p>
		 * 用于控制线程的阻塞/唤醒，以及避免不必要的调用LockSupport的park/unpark方法。
		 * CANCELLED=1、初始化=0、SIGNAL=-1、CONDITION=-2或PROPAGATE=-3。
		 */
		volatile int waitStatus;

		/**
		 * Link to predecessor node that current node/thread relies on for
		 * checking waitStatus. Assigned during enqueing, and nulled out (for
		 * sake of GC) only upon dequeuing. Also, upon cancellation of a
		 * predecessor, we short-circuit while finding a non-cancelled one,
		 * which will always exist because the head node is never cancelled: A
		 * node becomes head only as a result of successful acquire. A cancelled
		 * thread never succeeds in acquiring, and a thread only cancels itself,
		 * not any other node.
		 */
		volatile Node prev;

		/**
		 * Link to the successor node that the current node/thread unparks upon
		 * release. Assigned during enqueuing, adjusted when bypassing cancelled
		 * predecessors, and nulled out (for sake of GC) when dequeued. The enq
		 * operation does not assign next field of a predecessor until after
		 * attachment, so seeing a null next field does not necessarily mean
		 * that node is at end of queue. However, if a next field appears to be
		 * null, we can scan prev's from the tail to double-check. The next
		 * field of cancelled nodes is set to point to the node itself instead
		 * of null, to make life easier for isOnSyncQueue.
		 */
		volatile Node next;

		/**
		 * The thread that enqueued this node. Initialized on construction and
		 * nulled out after use.
		 */
		volatile Thread thread;

		/**
		 * Link to next node waiting on condition, or the special value SHARED.
		 * Because condition queues are accessed only when holding in exclusive
		 * mode, we just need a simple linked queue to hold nodes while they are
		 * waiting on conditions. They are then transferred to the queue to
		 * re-acquire. And because conditions can only be exclusive, we save a
		 * field by using special value to indicate shared mode.
		 */
		Node nextWaiter;

		/**
		 * Returns true if node is waiting in shared mode
		 */
		final boolean isShared() {
			return nextWaiter == SHARED;
		}

		/**
		 * Returns previous node, or throws NullPointerException if null. Use
		 * when predecessor cannot be null. The null check could be elided, but
		 * is present to help the VM.
		 * 
		 * @return the predecessor of this node
		 */
		final Node predecessor() throws NullPointerException {
			Node p = prev;
			if (p == null)
				throw new NullPointerException();
			else
				return p;
		}

		Node() { // Used to establish initial head or SHARED marker
		}

		Node(Thread thread, Node mode) { // Used by addWaiter
			this.nextWaiter = mode;
			this.thread = thread;
		}

		Node(Thread thread, int waitStatus) { // Used by Condition
			this.waitStatus = waitStatus;
			this.thread = thread;
		}
	}

	/**
	 * Head of the wait queue, lazily initialized. Except for initialization, it
	 * is modified only via method setHead. Note: If head exists, its waitStatus
	 * is guaranteed not to be CANCELLED.
	 */
	private transient volatile Node head;

	/**
	 * Tail of the wait queue, lazily initialized. Modified only via method enq
	 * to add new wait node.
	 */
	private transient volatile Node tail;

	/**
	 * The synchronization state.
	 * <p>
	 * 同步状态。
	 */
	private volatile int state;

	/**
	 * Returns the current value of synchronization state. This operation has
	 * memory semantics of a <tt>volatile</tt> read.
	 * 
	 * @return current state value
	 */
	protected final int getState() {
		return state;
	}

	/**
	 * Sets the value of synchronization state. This operation has memory
	 * semantics of a <tt>volatile</tt> write.
	 * 
	 * @param newState
	 *            the new state value
	 */
	protected final void setState(int newState) {
		state = newState;
	}

	/**
	 * Atomically sets synchronization state to the given updated value if the
	 * current state value equals the expected value. This operation has memory
	 * semantics of a <tt>volatile</tt> read and write.
	 * 
	 * @param expect
	 *            the expected value
	 * @param update
	 *            the new value
	 * @return true if successful. False return indicates that the actual value
	 *         was not equal to the expected value.
	 */
	protected final boolean compareAndSetState(int expect, int update) {
		// See below for intrinsics setup to support this
		return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
	}

	// Queuing utilities

	/**
	 * The number of nanoseconds for which it is faster to spin rather than to
	 * use timed park. A rough estimate suffices to improve responsiveness with
	 * very short timeouts.
	 */
	static final long spinForTimeoutThreshold = 1000L;

	/**
	 * Inserts node into queue, initializing if necessary. See picture above.
	 * <p>
	 * 入队操作。
	 * 
	 * @param node
	 *            the node to insert
	 * @return node's predecessor
	 */
	private Node enq(final Node node) {
		for (;;) {
			Node t = tail;
			if (t == null) { // Must initialize
				if (compareAndSetHead(new Node()))
					tail = head;
			} else {
				node.prev = t;
				if (compareAndSetTail(t, node)) {
					t.next = node;
					return t;
				}
			}
		}
	}

	/**
	 * Creates and enqueues node for current thread and given mode.
	 * <p>
	 * 入队操作
	 * 
	 * @param mode
	 *            Node.EXCLUSIVE for exclusive, Node.SHARED for shared
	 * @return the new node
	 */
	private Node addWaiter(Node mode) {
		Node node = new Node(Thread.currentThread(), mode);
		// Try the fast path of enq; backup to full enq on failure
		Node pred = tail;
		// 这个if分支其实是一种优化：CAS操作失败的话才进入enq中的循环。
		if (pred != null) {
			node.prev = pred;
			if (compareAndSetTail(pred, node)) {
				pred.next = node;
				return node;
			}
		}
		enq(node);
		return node;
	}

	/**
	 * Sets head of queue to be node, thus dequeuing. Called only by acquire
	 * methods. Also nulls out unused fields for sake of GC and to suppress
	 * unnecessary signals and traversals.
	 * <p>
	 * 出队操作。
	 * 
	 * @param node
	 *            the node
	 */
	private void setHead(Node node) {
		head = node;
		node.thread = null;
		node.prev = null;
	}

	/**
	 * Wakes up node's successor, if one exists.
	 * <p>
	 * 唤醒后续的结点
	 * 
	 * @param node
	 *            the node 其实 node == head
	 */
	private void unparkSuccessor(Node node) {
		/*
		 * If status is negative (i.e., possibly needing signal) try to clear in
		 * anticipation of signalling. It is OK if this fails or if status is
		 * changed by waiting thread.
		 */
		int ws = node.waitStatus;
		if (ws < 0)
			compareAndSetWaitStatus(node, ws, 0);

		/**
		 * Thread to unpark is held in successor, which is normally just the
		 * next node. But if cancelled or apparently null, traverse backwards
		 * from tail to find the actual non-cancelled successor.
		 * <p>
		 * 正常来说需要唤醒结点就是next，但如果next被取消了或者为null，<br />
		 * 则需要通过tail指针从后往前遍历找到没被取消的结点。
		 */
		Node s = node.next;
		// s == null || s.waitStatus == CANCELLED
		if (s == null || s.waitStatus > 0) {
			s = null;
			for (Node t = tail; t != null && t != node; t = t.prev)
				if (t.waitStatus <= 0)
					s = t;
		}
		if (s != null)
			// 唤醒后继
			LockSupport.unpark(s.thread);
	}

	/**
	 * Release action for shared mode -- signal successor and ensure
	 * propagation. (Note: For exclusive mode, release just amounts to calling
	 * unparkSuccessor of head if it needs signal.)
	 */
	private void doReleaseShared() {
		/**
		 * Ensure that a release propagates, even if there are other in-progress
		 * acquires/releases. This proceeds in the usual way of trying to
		 * unparkSuccessor of head if it needs signal. But if it does not,
		 * status is set to PROPAGATE to ensure that upon release, propagation
		 * continues. Additionally, we must loop in case a new node is added
		 * while we are doing this. Also, unlike other uses of unparkSuccessor,
		 * we need to know if CAS to reset status fails, if so rechecking.
		 * <p>
		 */
		for (;;) {
			Node h = head;
			// 队列不为空且有后继结点
			if (h != null && h != tail) {
				int ws = h.waitStatus;
				// 不管是共享还是独占只有结点状态为SIGNAL才尝试唤醒后继结点
				if (ws == Node.SIGNAL) {
					// 将waitStatus设置为0
					if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
						continue; // loop to recheck cases
					unparkSuccessor(h);// 唤醒后继结点
					// 如果状态为0则更新状态为PROPAGATE，更新失败则重试
				} else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
					continue; // loop on failed CAS
			}
			// 如果过程中head被修改了则重试。
			if (h == head) // loop if head changed
				break;
		}
	}

	/**
	 * Sets head of queue, and checks if successor may be waiting in shared
	 * mode, if so propagating if either propagate > 0 or PROPAGATE status was
	 * set.
	 * <p>
	 * 将node设置为head。如果当前结点acquire到了之后发现还有许可可以被获取，则继续释放自己的后继，
	 * 后继会将这个操作传递下去。这就是PROPAGATE状态的含义。
	 * 
	 * @param node
	 *            the node
	 * @param propagate
	 *            the return value from a tryAcquireShared
	 */
	private void setHeadAndPropagate(Node node, int propagate) {
		Node h = head; // Record old head for check below
		setHead(node);
		/**
		 * Try to signal next queued node if: Propagation was indicated by
		 * caller, or was recorded (as h.waitStatus) by a previous operation
		 * (note: this uses sign-check of waitStatus because PROPAGATE status
		 * may transition to SIGNAL.) and The next node is waiting in shared
		 * mode, or we don't know, because it appears null
		 * 
		 * The conservatism in both of these checks may cause unnecessary
		 * wake-ups, but only when there are multiple racing acquires/releases,
		 * so most need signals now or soon anyway.
		 * <p>
		 * 尝试唤醒后继的结点：<br />
		 * propagate > 0说明许可还有能够继续被线程acquire;<br />
		 * 或者 之前的head被设置为PROPAGATE(PROPAGATE可以被转换为SIGNAL)说明需要往后传递;<br />
		 * 或者为null,我们还不确定什么情况。 <br />
		 * 并且 后继结点是共享模式或者为如上为null。
		 * <p>
		 * 上面的检查有点保守，在有多个线程竞争获取/释放的时候可能会导致不必要的唤醒。<br />
		 * 
		 */
		if (propagate > 0 || h == null || h.waitStatus < 0) {
			Node s = node.next;
			// 后继结是共享模式或者s == null（不知道什么情况）
			// 如果后继是独占模式，那么即使剩下的许可大于0也不会继续往后传递唤醒操作
			// 即使后面有结点是共享模式。
			if (s == null || s.isShared())
				// 唤醒后继结点
				doReleaseShared();
		}
	}

	// Utilities for various versions of acquire

	/**
	 * Cancels an ongoing attempt to acquire.
	 * 
	 * @param node
	 *            the node
	 */
	private void cancelAcquire(Node node) {
		// Ignore if node doesn't exist
		if (node == null)
			return;

		node.thread = null;

		// Skip cancelled predecessors
		Node pred = node.prev;
		while (pred.waitStatus > 0)
			node.prev = pred = pred.prev;

		// predNext is the apparent node to unsplice. CASes below will
		// fail if not, in which case, we lost race vs another cancel
		// or signal, so no further action is necessary.
		Node predNext = pred.next;

		// Can use unconditional write instead of CAS here.
		// After this atomic step, other Nodes can skip past us.
		// Before, we are free of interference from other threads.
		node.waitStatus = Node.CANCELLED;

		// If we are the tail, remove ourselves.
		if (node == tail && compareAndSetTail(node, pred)) {
			compareAndSetNext(pred, predNext, null);
		} else {
			// If successor needs signal, try to set pred's next-link
			// so it will get one. Otherwise wake it up to propagate.
			int ws;
			if (pred != head
					&& ((ws = pred.waitStatus) == Node.SIGNAL || (ws <= 0 && compareAndSetWaitStatus(pred, ws,
							Node.SIGNAL))) && pred.thread != null) {
				Node next = node.next;
				if (next != null && next.waitStatus <= 0)
					compareAndSetNext(pred, predNext, next);
			} else {
				unparkSuccessor(node);
			}

			node.next = node; // help GC
		}
	}

	/**
	 * Checks and updates status for a node that failed to acquire. Returns true
	 * if thread should block. This is the main signal control in all acquire
	 * loops. Requires that pred == node.prev
	 * <p>
	 * 1.确定后继是否需要park;<br />
	 * 2.跳过被取消的结点;<br />
	 * 3.设置前继的waitStatus为SIGNAL.
	 * 
	 * @param pred
	 *            node's predecessor holding status
	 * @param node
	 *            the node
	 * @return {@code true} if thread should block
	 */
	private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
		int ws = pred.waitStatus;
		if (ws == Node.SIGNAL)// 前继结点已经准备好unpark其后继了，所以后继可以安全的park
			/*
			 * This node has already set status asking a release to signal it,
			 * so it can safely park.
			 */
			return true;
		if (ws > 0) {// CANCELLED
			/*
			 * Predecessor was cancelled. Skip over predecessors and indicate
			 * retry. 跳过被取消的结点。
			 */
			do {
				node.prev = pred = pred.prev;
			} while (pred.waitStatus > 0);
			pred.next = node;
		} else {// 0 、PROPAGATE、CONDITION
			/**
			 * waitStatus must be 0 or PROPAGATE. Indicate that we need a
			 * signal, but don't park yet. Caller will need to retry to make
			 * sure it cannot acquire before parking.
			 * <p>
			 * waitStatus 等于0（初始化）或PROPAGATE。说明线程还没有park，会先重试 确定无法acquire到再park。
			 */

			// 更新pred结点waitStatus为SIGNAL
			compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
		}
		return false;
	}

	/**
	 * Convenience method to interrupt current thread.
	 */
	private static void selfInterrupt() {
		Thread.currentThread().interrupt();
	}

	/**
	 * Convenience method to park and then check if interrupted
	 * <p>
	 * 该方法返回有3种情况：
	 * <ul>
	 * <li>线程被中断</li>
	 * <li>线程被unpark</li>
	 * <li>伪唤醒</li>
	 * </ul>
	 * 
	 * @return {@code true} if interrupted
	 */
	private final boolean parkAndCheckInterrupt() {
		LockSupport.park(this);
		return Thread.interrupted();
	}

	/*
	 * Various flavors of acquire, varying in exclusive/shared and control
	 * modes. Each is mostly the same, but annoyingly different. Only a little
	 * bit of factoring is possible due to interactions of exception mechanics
	 * (including ensuring that we cancel if tryAcquire throws exception) and
	 * other control, at least not without hurting performance too much.
	 */

	/**
	 * Acquires in exclusive uninterruptible mode for thread already in queue.
	 * Used by condition wait methods as well as acquire.
	 * <p>
	 * 等待前继结点释放锁，忽视线程的中断，直到tryAcquire成功。
	 * 
	 * @param node
	 *            the node
	 * @param arg
	 *            the acquire argument
	 * @return {@code true} if interrupted while waiting
	 */
	final boolean acquireQueued(final Node node, int arg) {
		boolean failed = true;
		try {
			boolean interrupted = false;
			// 等待前继结点释放锁
			// 自旋re-check
			for (;;) {
				// 获取前继
				final Node p = node.predecessor();
				// 前继是head,说明next就是node了，则尝试获取锁。
				if (p == head && tryAcquire(arg)) {
					// 前继出队，node成为head
					setHead(node);
					p.next = null; // help GC
					failed = false;
					return interrupted;
				}

				// p != head 或者 p == head但是tryAcquire失败了，那么
				// 应该阻塞当前线程等待前继唤醒。阻塞之前会再重试一次，还需要设置前继的waitStaus为SIGNAL。

				// 线程会阻塞在parkAndCheckInterrupt方法中。
				// parkAndCheckInterrupt返回可能是前继unpark或线程被中断。
				if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
					// 说明当前线程是被中断唤醒的。
					// 注意：线程被中断之后会继续走到if处去判断，也就是会忽视中断。
					// 除非碰巧线程中断后acquire成功了，那么根据Java的最佳实践，
					// 需要重新设置线程的中断状态（acquire.selfInterrupt）。
					interrupted = true;
			}
		} finally {
			// 出现异常
			if (failed)
				cancelAcquire(node);
		}
	}

	/**
	 * Acquires in exclusive interruptible mode.
	 * 
	 * @param arg
	 *            the acquire argument
	 */
	private void doAcquireInterruptibly(int arg) throws InterruptedException {
		final Node node = addWaiter(Node.EXCLUSIVE);
		boolean failed = true;
		try {
			for (;;) {
				final Node p = node.predecessor();
				if (p == head && tryAcquire(arg)) {
					setHead(node);
					p.next = null; // help GC
					failed = false;
					return;
				}
				if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
					throw new InterruptedException();
			}
		} finally {
			if (failed)
				cancelAcquire(node);
		}
	}

	/**
	 * Acquires in exclusive timed mode.
	 * <p>
	 * 代码整体上跟{@link #acquireQueued}差不多。
	 * 
	 * @param arg
	 *            the acquire argument
	 * @param nanosTimeout
	 *            max wait time
	 * @return {@code true} if acquired
	 */
	private boolean doAcquireNanos(int arg, long nanosTimeout) throws InterruptedException {
		long lastTime = System.nanoTime();
		final Node node = addWaiter(Node.EXCLUSIVE);
		boolean failed = true;
		try {
			for (;;) {
				final Node p = node.predecessor();
				if (p == head && tryAcquire(arg)) {
					setHead(node);
					p.next = null; // help GC
					failed = false;
					return true;
				}
				if (nanosTimeout <= 0)// 超时
					return false;
				// nanosTimeout > spinForTimeoutThreshold
				// 如果超时时间很短的话，自旋效率会更高。
				if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold)
					LockSupport.parkNanos(this, nanosTimeout);
				long now = System.nanoTime();
				nanosTimeout -= now - lastTime;
				lastTime = now;
				if (Thread.interrupted())
					throw new InterruptedException();
			}
		} finally {
			if (failed)
				cancelAcquire(node);
		}
	}

	/**
	 * Acquires in shared uninterruptible mode.
	 * 
	 * @param arg
	 *            the acquire argument
	 */
	private void doAcquireShared(int arg) {
		// 添加队列
		final Node node = addWaiter(Node.SHARED);
		boolean failed = true;
		try {
			boolean interrupted = false;
			// 等待前继释放并传递
			for (;;) {
				final Node p = node.predecessor();
				if (p == head) {
					int r = tryAcquireShared(arg);// 尝试获取
					if (r >= 0) {
						// 获取成功则前继出队，跟独占不同的是
						// 会往后面结点传播唤醒的操作，保证剩下等待的线程能够尽快获取到剩下的许可。
						setHeadAndPropagate(node, r);
						p.next = null; // help GC
						if (interrupted)
							selfInterrupt();
						failed = false;
						return;
					}
				}

				// p != head || r < 0
				if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
					interrupted = true;
			}
		} finally {
			if (failed)
				cancelAcquire(node);
		}
	}

	/**
	 * Acquires in shared interruptible mode.
	 * 
	 * @param arg
	 *            the acquire argument
	 */
	private void doAcquireSharedInterruptibly(int arg) throws InterruptedException {
		final Node node = addWaiter(Node.SHARED);
		boolean failed = true;
		try {
			for (;;) {
				final Node p = node.predecessor();
				if (p == head) {
					int r = tryAcquireShared(arg);
					if (r >= 0) {
						setHeadAndPropagate(node, r);
						p.next = null; // help GC
						failed = false;
						return;
					}
				}
				if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
					throw new InterruptedException();
			}
		} finally {
			if (failed)
				cancelAcquire(node);
		}
	}

	/**
	 * Acquires in shared timed mode.
	 * 
	 * @param arg
	 *            the acquire argument
	 * @param nanosTimeout
	 *            max wait time
	 * @return {@code true} if acquired
	 */
	private boolean doAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException {

		long lastTime = System.nanoTime();
		final Node node = addWaiter(Node.SHARED);
		boolean failed = true;
		try {
			for (;;) {
				final Node p = node.predecessor();
				if (p == head) {
					int r = tryAcquireShared(arg);
					if (r >= 0) {
						setHeadAndPropagate(node, r);
						p.next = null; // help GC
						failed = false;
						return true;
					}
				}
				if (nanosTimeout <= 0)
					return false;
				if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold)
					LockSupport.parkNanos(this, nanosTimeout);
				long now = System.nanoTime();
				nanosTimeout -= now - lastTime;
				lastTime = now;
				if (Thread.interrupted())
					throw new InterruptedException();
			}
		} finally {
			if (failed)
				cancelAcquire(node);
		}
	}

	// Main exported methods

	/**
	 * Attempts to acquire in exclusive mode. This method should query if the
	 * state of the object permits it to be acquired in the exclusive mode, and
	 * if so to acquire it.
	 * 
	 * <p>
	 * This method is always invoked by the thread performing acquire. If this
	 * method reports failure, the acquire method may queue the thread, if it is
	 * not already queued, until it is signalled by a release from some other
	 * thread. This can be used to implement method {@link Lock#tryLock()}.
	 * 
	 * <p>
	 * The default implementation throws {@link UnsupportedOperationException}.
	 * 
	 * @param arg
	 *            the acquire argument. This value is always the one passed to
	 *            an acquire method, or is the value saved on entry to a
	 *            condition wait. The value is otherwise uninterpreted and can
	 *            represent anything you like.
	 * @return {@code true} if successful. Upon success, this object has been
	 *         acquired.
	 * @throws IllegalMonitorStateException
	 *             if acquiring would place this synchronizer in an illegal
	 *             state. This exception must be thrown in a consistent fashion
	 *             for synchronization to work correctly.
	 * @throws UnsupportedOperationException
	 *             if exclusive mode is not supported
	 */
	protected boolean tryAcquire(int arg) {
		throw new UnsupportedOperationException();
	}

	/**
	 * Attempts to set the state to reflect a release in exclusive mode.
	 * 
	 * <p>
	 * This method is always invoked by the thread performing release.
	 * 
	 * <p>
	 * The default implementation throws {@link UnsupportedOperationException}.
	 * 
	 * @param arg
	 *            the release argument. This value is always the one passed to a
	 *            release method, or the current state value upon entry to a
	 *            condition wait. The value is otherwise uninterpreted and can
	 *            represent anything you like.
	 * @return {@code true} if this object is now in a fully released state, so
	 *         that any waiting threads may attempt to acquire; and
	 *         {@code false} otherwise.
	 * @throws IllegalMonitorStateException
	 *             if releasing would place this synchronizer in an illegal
	 *             state. This exception must be thrown in a consistent fashion
	 *             for synchronization to work correctly.
	 * @throws UnsupportedOperationException
	 *             if exclusive mode is not supported
	 */
	protected boolean tryRelease(int arg) {
		throw new UnsupportedOperationException();
	}

	/**
	 * Attempts to acquire in shared mode. This method should query if the state
	 * of the object permits it to be acquired in the shared mode, and if so to
	 * acquire it.
	 * 
	 * <p>
	 * This method is always invoked by the thread performing acquire. If this
	 * method reports failure, the acquire method may queue the thread, if it is
	 * not already queued, until it is signalled by a release from some other
	 * thread.
	 * 
	 * <p>
	 * The default implementation throws {@link UnsupportedOperationException}.
	 * 
	 * @param arg
	 *            the acquire argument. This value is always the one passed to
	 *            an acquire method, or is the value saved on entry to a
	 *            condition wait. The value is otherwise uninterpreted and can
	 *            represent anything you like.
	 * @return a negative value on failure; zero if acquisition in shared mode
	 *         succeeded but no subsequent shared-mode acquire can succeed; and
	 *         a positive value if acquisition in shared mode succeeded and
	 *         subsequent shared-mode acquires might also succeed, in which case
	 *         a subsequent waiting thread must check availability. (Support for
	 *         three different return values enables this method to be used in
	 *         contexts where acquires only sometimes act exclusively.) Upon
	 *         success, this object has been acquired.
	 * @throws IllegalMonitorStateException
	 *             if acquiring would place this synchronizer in an illegal
	 *             state. This exception must be thrown in a consistent fashion
	 *             for synchronization to work correctly.
	 * @throws UnsupportedOperationException
	 *             if shared mode is not supported
	 */
	protected int tryAcquireShared(int arg) {
		throw new UnsupportedOperationException();
	}

	/**
	 * Attempts to set the state to reflect a release in shared mode.
	 * 
	 * <p>
	 * This method is always invoked by the thread performing release.
	 * 
	 * <p>
	 * The default implementation throws {@link UnsupportedOperationException}.
	 * 
	 * @param arg
	 *            the release argument. This value is always the one passed to a
	 *            release method, or the current state value upon entry to a
	 *            condition wait. The value is otherwise uninterpreted and can
	 *            represent anything you like.
	 * @return {@code true} if this release of shared mode may permit a waiting
	 *         acquire (shared or exclusive) to succeed; and {@code false}
	 *         otherwise
	 * @throws IllegalMonitorStateException
	 *             if releasing would place this synchronizer in an illegal
	 *             state. This exception must be thrown in a consistent fashion
	 *             for synchronization to work correctly.
	 * @throws UnsupportedOperationException
	 *             if shared mode is not supported
	 */
	protected boolean tryReleaseShared(int arg) {
		throw new UnsupportedOperationException();
	}

	/**
	 * Returns {@code true} if synchronization is held exclusively with respect
	 * to the current (calling) thread. This method is invoked upon each call to
	 * a non-waiting {@link ConditionObject} method. (Waiting methods instead
	 * invoke {@link #release}.)
	 * 
	 * <p>
	 * The default implementation throws {@link UnsupportedOperationException}.
	 * This method is invoked internally only within {@link ConditionObject}
	 * methods, so need not be defined if conditions are not used.
	 * 
	 * @return {@code true} if synchronization is held exclusively;
	 *         {@code false} otherwise
	 * @throws UnsupportedOperationException
	 *             if conditions are not supported
	 */
	protected boolean isHeldExclusively() {
		throw new UnsupportedOperationException();
	}

	/**
	 * Acquires in exclusive mode, ignoring interrupts. Implemented by invoking
	 * at least once {@link #tryAcquire}, returning on success. Otherwise the
	 * thread is queued, possibly repeatedly blocking and unblocking, invoking
	 * {@link #tryAcquire} until success. This method can be used to implement
	 * method {@link Lock#lock}.
	 * <p>
	 * 独占获取。
	 * <p>
	 * 一旦tryAcquire成功则立即返回，否则线程会加入队列。 线程可能会反复的被阻塞和唤醒直到tryAcquire成功，这是因为线程可能被中断，
	 * 而acquireQueued方法中会保证忽视中断，只有tryAcquire成功了才返回。
	 * <p>
	 * 可以中断的版本{@link #acquireInterruptibly} ，中断时会抛出InterruptedException异常。
	 * 
	 * 
	 * 
	 * @param arg
	 *            the acquire argument. This value is conveyed to
	 *            {@link #tryAcquire} but is otherwise uninterpreted and can
	 *            represent anything you like.
	 */
	public final void acquire(int arg) {
		// tryAcquire 由子类实现本身不会阻塞线程，如果返回 true,则线程继续，
		// 如果返回 false 那么就 加入阻塞队列阻塞线程，并等待前继结点释放锁。
		if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
			// acquireQueued返回true，说明当前线程被中断唤醒后获取到锁，
			// 重置其interrupt status为true。
			selfInterrupt();
	}

	/**
	 * Acquires in exclusive mode, aborting if interrupted. Implemented by first
	 * checking interrupt status, then invoking at least once
	 * {@link #tryAcquire}, returning on success. Otherwise the thread is
	 * queued, possibly repeatedly blocking and unblocking, invoking
	 * {@link #tryAcquire} until success or the thread is interrupted. This
	 * method can be used to implement method {@link Lock#lockInterruptibly}.
	 * <p>
	 * 独占且可中断模式获取：支持中断取消。
	 * 
	 * @param arg
	 *            the acquire argument. This value is conveyed to
	 *            {@link #tryAcquire} but is otherwise uninterpreted and can
	 *            represent anything you like.
	 * @throws InterruptedException
	 *             if the current thread is interrupted
	 */
	public final void acquireInterruptibly(int arg) throws InterruptedException {
		if (Thread.interrupted())
			throw new InterruptedException();
		if (!tryAcquire(arg))
			doAcquireInterruptibly(arg);
	}

	/**
	 * Attempts to acquire in exclusive mode, aborting if interrupted, and
	 * failing if the given timeout elapses. Implemented by first checking
	 * interrupt status, then invoking at least once {@link #tryAcquire},
	 * returning on success. Otherwise, the thread is queued, possibly
	 * repeatedly blocking and unblocking, invoking {@link #tryAcquire} until
	 * success or the thread is interrupted or the timeout elapses. This method
	 * can be used to implement method {@link Lock#tryLock(long, TimeUnit)}.
	 * <p>
	 * 独占且支持超时模式获取： 带有超时时间，如果超过超时时间则会退出。
	 * 
	 * @param arg
	 *            the acquire argument. This value is conveyed to
	 *            {@link #tryAcquire} but is otherwise uninterpreted and can
	 *            represent anything you like.
	 * @param nanosTimeout
	 *            the maximum number of nanoseconds to wait
	 * @return {@code true} if acquired; {@code false} if timed out
	 * @throws InterruptedException
	 *             if the current thread is interrupted
	 */
	public final boolean tryAcquireNanos(int arg, long nanosTimeout) throws InterruptedException {
		if (Thread.interrupted())
			throw new InterruptedException();
		return tryAcquire(arg) || doAcquireNanos(arg, nanosTimeout);
	}

	/**
	 * Releases in exclusive mode. Implemented by unblocking one or more threads
	 * if {@link #tryRelease} returns true. This method can be used to implement
	 * method {@link Lock#unlock}.
	 * <p>
	 * 独占模式释放：释放成功会唤醒后续节点。
	 * 
	 * @param arg
	 *            the release argument. This value is conveyed to
	 *            {@link #tryRelease} but is otherwise uninterpreted and can
	 *            represent anything you like.
	 * @return the value returned from {@link #tryRelease}
	 */
	public final boolean release(int arg) {
		// tryReease由子类实现，通过设置state值来达到同步的效果。
		if (tryRelease(arg)) {
			Node h = head;
			// waitStatus为0说明是初始化的空队列
			if (h != null && h.waitStatus != 0)
				// 唤醒后续的结点
				unparkSuccessor(h);
			return true;
		}
		return false;
	}

	/**
	 * Acquires in shared mode, ignoring interrupts. Implemented by first
	 * invoking at least once {@link #tryAcquireShared}, returning on success.
	 * Otherwise the thread is queued, possibly repeatedly blocking and
	 * unblocking, invoking {@link #tryAcquireShared} until success.
	 * <p>
	 * 共享模式获取。
	 * 
	 * @param arg
	 *            the acquire argument. This value is conveyed to
	 *            {@link #tryAcquireShared} but is otherwise uninterpreted and
	 *            can represent anything you like.
	 */
	public final void acquireShared(int arg) {
		//如果没有许可了则入队等待
		if (tryAcquireShared(arg) < 0)
			doAcquireShared(arg);
	}

	/**
	 * Acquires in shared mode, aborting if interrupted. Implemented by first
	 * checking interrupt status, then invoking at least once
	 * {@link #tryAcquireShared}, returning on success. Otherwise the thread is
	 * queued, possibly repeatedly blocking and unblocking, invoking
	 * {@link #tryAcquireShared} until success or the thread is interrupted.
	 * <p>
	 * 可中断模式共享获取。
	 * 
	 * @param arg
	 *            the acquire argument This value is conveyed to
	 *            {@link #tryAcquireShared} but is otherwise uninterpreted and
	 *            can represent anything you like.
	 * @throws InterruptedException
	 *             if the current thread is interrupted
	 */
	public final void acquireSharedInterruptibly(int arg) throws InterruptedException {
		if (Thread.interrupted())
			throw new InterruptedException();
		if (tryAcquireShared(arg) < 0)
			doAcquireSharedInterruptibly(arg);
	}

	/**
	 * Attempts to acquire in shared mode, aborting if interrupted, and failing
	 * if the given timeout elapses. Implemented by first checking interrupt
	 * status, then invoking at least once {@link #tryAcquireShared}, returning
	 * on success. Otherwise, the thread is queued, possibly repeatedly blocking
	 * and unblocking, invoking {@link #tryAcquireShared} until success or the
	 * thread is interrupted or the timeout elapses.
	 * <p>
	 * 共享模式带超时。
	 * 
	 * @param arg
	 *            the acquire argument. This value is conveyed to
	 *            {@link #tryAcquireShared} but is otherwise uninterpreted and
	 *            can represent anything you like.
	 * @param nanosTimeout
	 *            the maximum number of nanoseconds to wait
	 * @return {@code true} if acquired; {@code false} if timed out
	 * @throws InterruptedException
	 *             if the current thread is interrupted
	 */
	public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException {
		if (Thread.interrupted())
			throw new InterruptedException();
		return tryAcquireShared(arg) >= 0 || doAcquireSharedNanos(arg, nanosTimeout);
	}

	/**
	 * Releases in shared mode. Implemented by unblocking one or more threads if
	 * {@link #tryReleaseShared} returns true.
	 * <p>
	 * 共享锁释放。
	 * 
	 * @param arg
	 *            the release argument. This value is conveyed to
	 *            {@link #tryReleaseShared} but is otherwise uninterpreted and
	 *            can represent anything you like.
	 * @return the value returned from {@link #tryReleaseShared}
	 */
	public final boolean releaseShared(int arg) {
		if (tryReleaseShared(arg)) {
			doReleaseShared();
			return true;
		}
		return false;
	}

	// Queue inspection methods

	/**
	 * Queries whether any threads are waiting to acquire. Note that because
	 * cancellations due to interrupts and timeouts may occur at any time, a
	 * {@code true} return does not guarantee that any other thread will ever
	 * acquire.
	 * 
	 * <p>
	 * In this implementation, this operation returns in constant time.
	 * 
	 * @return {@code true} if there may be other threads waiting to acquire
	 */
	public final boolean hasQueuedThreads() {
		return head != tail;
	}

	/**
	 * Queries whether any threads have ever contended to acquire this
	 * synchronizer; that is if an acquire method has ever blocked.
	 * 
	 * <p>
	 * In this implementation, this operation returns in constant time.
	 * 
	 * @return {@code true} if there has ever been contention
	 */
	public final boolean hasContended() {
		return head != null;
	}

	/**
	 * Returns the first (longest-waiting) thread in the queue, or {@code null}
	 * if no threads are currently queued.
	 * 
	 * <p>
	 * In this implementation, this operation normally returns in constant time,
	 * but may iterate upon contention if other threads are concurrently
	 * modifying the queue.
	 * 
	 * @return the first (longest-waiting) thread in the queue, or {@code null}
	 *         if no threads are currently queued
	 */
	public final Thread getFirstQueuedThread() {
		// handle only fast path, else relay
		return (head == tail) ? null : fullGetFirstQueuedThread();
	}

	/**
	 * Version of getFirstQueuedThread called when fastpath fails
	 */
	private Thread fullGetFirstQueuedThread() {
		/*
		 * The first node is normally head.next. Try to get its thread field,
		 * ensuring consistent reads: If thread field is nulled out or s.prev is
		 * no longer head, then some other thread(s) concurrently performed
		 * setHead in between some of our reads. We try this twice before
		 * resorting to traversal.
		 */
		Node h, s;
		Thread st;
		if (((h = head) != null && (s = h.next) != null && s.prev == head && (st = s.thread) != null)
				|| ((h = head) != null && (s = h.next) != null && s.prev == head && (st = s.thread) != null))
			return st;

		/*
		 * Head's next field might not have been set yet, or may have been unset
		 * after setHead. So we must check to see if tail is actually first
		 * node. If not, we continue on, safely traversing from tail back to
		 * head to find first, guaranteeing termination.
		 */

		Node t = tail;
		Thread firstThread = null;
		while (t != null && t != head) {
			Thread tt = t.thread;
			if (tt != null)
				firstThread = tt;
			t = t.prev;
		}
		return firstThread;
	}

	/**
	 * Returns true if the given thread is currently queued.
	 * 
	 * <p>
	 * This implementation traverses the queue to determine presence of the
	 * given thread.
	 * 
	 * @param thread
	 *            the thread
	 * @return {@code true} if the given thread is on the queue
	 * @throws NullPointerException
	 *             if the thread is null
	 */
	public final boolean isQueued(Thread thread) {
		if (thread == null)
			throw new NullPointerException();
		for (Node p = tail; p != null; p = p.prev)
			if (p.thread == thread)
				return true;
		return false;
	}

	/**
	 * Returns {@code true} if the apparent first queued thread, if one exists,
	 * is waiting in exclusive mode. If this method returns {@code true}, and
	 * the current thread is attempting to acquire in shared mode (that is, this
	 * method is invoked from {@link #tryAcquireShared}) then it is guaranteed
	 * that the current thread is not the first queued thread. Used only as a
	 * heuristic in ReentrantReadWriteLock.
	 */
	final boolean apparentlyFirstQueuedIsExclusive() {
		Node h, s;
		return (h = head) != null && (s = h.next) != null && !s.isShared() && s.thread != null;
	}

	/**
	 * Queries whether any threads have been waiting to acquire longer than the
	 * current thread.
	 * 
	 * <p>
	 * An invocation of this method is equivalent to (but may be more efficient
	 * than):
	 * 
	 * <pre>
	 * {@code
	 * getFirstQueuedThread() != Thread.currentThread() &&
	 * hasQueuedThreads()}
	 * </pre>
	 * 
	 * <p>
	 * Note that because cancellations due to interrupts and timeouts may occur
	 * at any time, a {@code true} return does not guarantee that some other
	 * thread will acquire before the current thread. Likewise, it is possible
	 * for another thread to win a race to enqueue after this method has
	 * returned {@code false}, due to the queue being empty.
	 * 
	 * <p>
	 * This method is designed to be used by a fair synchronizer to avoid <a
	 * href="AbstractQueuedSynchronizer#barging">barging</a>. Such a
	 * synchronizer's {@link #tryAcquire} method should return {@code false},
	 * and its {@link #tryAcquireShared} method should return a negative value,
	 * if this method returns {@code true} (unless this is a reentrant acquire).
	 * For example, the {@code tryAcquire} method for a fair, reentrant,
	 * exclusive mode synchronizer might look like this:
	 * 
	 * <pre>
	 * {@code
	 * protected boolean tryAcquire(int arg) {
	 *   if (isHeldExclusively()) {
	 *     // A reentrant acquire; increment hold count
	 *     return true;
	 *   } else if (hasQueuedPredecessors()) {
	 *     return false;
	 *   } else {
	 *     // try to acquire normally
	 *   }
	 * }}
	 * </pre>
	 * 
	 * @return {@code true} if there is a queued thread preceding the current
	 *         thread, and {@code false} if the current thread is at the head of
	 *         the queue or the queue is empty
	 * @since 1.7
	 */
	public final boolean hasQueuedPredecessors() {
		// The correctness of this depends on head being initialized
		// before tail and on head.next being accurate if the current
		// thread is first in queue.
		Node t = tail; // Read fields in reverse initialization order
		Node h = head;
		Node s;
		return h != t && ((s = h.next) == null || s.thread != Thread.currentThread());
	}

	// Instrumentation and monitoring methods

	/**
	 * Returns an estimate of the number of threads waiting to acquire. The
	 * value is only an estimate because the number of threads may change
	 * dynamically while this method traverses internal data structures. This
	 * method is designed for use in monitoring system state, not for
	 * synchronization control.
	 * 
	 * @return the estimated number of threads waiting to acquire
	 */
	public final int getQueueLength() {
		int n = 0;
		for (Node p = tail; p != null; p = p.prev) {
			if (p.thread != null)
				++n;
		}
		return n;
	}

	/**
	 * Returns a collection containing threads that may be waiting to acquire.
	 * Because the actual set of threads may change dynamically while
	 * constructing this result, the returned collection is only a best-effort
	 * estimate. The elements of the returned collection are in no particular
	 * order. This method is designed to facilitate construction of subclasses
	 * that provide more extensive monitoring facilities.
	 * 
	 * @return the collection of threads
	 */
	public final Collection<Thread> getQueuedThreads() {
		ArrayList<Thread> list = new ArrayList<Thread>();
		for (Node p = tail; p != null; p = p.prev) {
			Thread t = p.thread;
			if (t != null)
				list.add(t);
		}
		return list;
	}

	/**
	 * Returns a collection containing threads that may be waiting to acquire in
	 * exclusive mode. This has the same properties as {@link #getQueuedThreads}
	 * except that it only returns those threads waiting due to an exclusive
	 * acquire.
	 * 
	 * @return the collection of threads
	 */
	public final Collection<Thread> getExclusiveQueuedThreads() {
		ArrayList<Thread> list = new ArrayList<Thread>();
		for (Node p = tail; p != null; p = p.prev) {
			if (!p.isShared()) {
				Thread t = p.thread;
				if (t != null)
					list.add(t);
			}
		}
		return list;
	}

	/**
	 * Returns a collection containing threads that may be waiting to acquire in
	 * shared mode. This has the same properties as {@link #getQueuedThreads}
	 * except that it only returns those threads waiting due to a shared
	 * acquire.
	 * 
	 * @return the collection of threads
	 */
	public final Collection<Thread> getSharedQueuedThreads() {
		ArrayList<Thread> list = new ArrayList<Thread>();
		for (Node p = tail; p != null; p = p.prev) {
			if (p.isShared()) {
				Thread t = p.thread;
				if (t != null)
					list.add(t);
			}
		}
		return list;
	}

	/**
	 * Returns a string identifying this synchronizer, as well as its state. The
	 * state, in brackets, includes the String {@code "State ="} followed by the
	 * current value of {@link #getState}, and either {@code "nonempty"} or
	 * {@code "empty"} depending on whether the queue is empty.
	 * 
	 * @return a string identifying this synchronizer, as well as its state
	 */
	public String toString() {
		int s = getState();
		String q = hasQueuedThreads() ? "non" : "";
		return super.toString() + "[State = " + s + ", " + q + "empty queue]";
	}

	// Internal support methods for Conditions

	/**
	 * Returns true if a node, always one that was initially placed on a
	 * condition queue, is now waiting to reacquire on sync queue.
	 * 
	 * @param node
	 *            the node
	 * @return true if is reacquiring
	 */
	final boolean isOnSyncQueue(Node node) {
		if (node.waitStatus == Node.CONDITION || node.prev == null)
			return false;
		if (node.next != null) // If has successor, it must be on queue
			return true;
		/*
		 * node.prev can be non-null, but not yet on queue because the CAS to
		 * place it on queue can fail. So we have to traverse from tail to make
		 * sure it actually made it. It will always be near the tail in calls to
		 * this method, and unless the CAS failed (which is unlikely), it will
		 * be there, so we hardly ever traverse much.
		 */
		return findNodeFromTail(node);
	}

	/**
	 * Returns true if node is on sync queue by searching backwards from tail.
	 * Called only when needed by isOnSyncQueue.
	 * 
	 * @return true if present
	 */
	private boolean findNodeFromTail(Node node) {
		Node t = tail;
		for (;;) {
			if (t == node)
				return true;
			if (t == null)
				return false;
			t = t.prev;
		}
	}

	/**
	 * Transfers a node from a condition queue onto sync queue. Returns true if
	 * successful.
	 * 
	 * @param node
	 *            the node
	 * @return true if successfully transferred (else the node was cancelled
	 *         before signal).
	 */
	final boolean transferForSignal(Node node) {
		/*
		 * If cannot change waitStatus, the node has been cancelled.
		 */
		if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
			return false;

		/*
		 * Splice onto queue and try to set waitStatus of predecessor to
		 * indicate that thread is (probably) waiting. If cancelled or attempt
		 * to set waitStatus fails, wake up to resync (in which case the
		 * waitStatus can be transiently and harmlessly wrong).
		 */
		Node p = enq(node);
		int ws = p.waitStatus;
		if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
			LockSupport.unpark(node.thread);
		return true;
	}

	/**
	 * Transfers node, if necessary, to sync queue after a cancelled wait.
	 * Returns true if thread was cancelled before being signalled.
	 * 
	 * @param current
	 *            the waiting thread
	 * @param node
	 *            its node
	 * @return true if cancelled before the node was signalled
	 */
	final boolean transferAfterCancelledWait(Node node) {
		if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
			enq(node);
			return true;
		}
		/*
		 * If we lost out to a signal(), then we can't proceed until it finishes
		 * its enq(). Cancelling during an incomplete transfer is both rare and
		 * transient, so just spin.
		 */
		while (!isOnSyncQueue(node))
			Thread.yield();
		return false;
	}

	/**
	 * Invokes release with current state value; returns saved state. Cancels
	 * node and throws exception on failure.
	 * 
	 * @param node
	 *            the condition node for this wait
	 * @return previous sync state
	 */
	final int fullyRelease(Node node) {
		boolean failed = true;
		try {
			int savedState = getState();
			if (release(savedState)) {
				failed = false;
				return savedState;
			} else {
				throw new IllegalMonitorStateException();
			}
		} finally {
			if (failed)
				node.waitStatus = Node.CANCELLED;
		}
	}

	// Instrumentation methods for conditions

	/**
	 * Queries whether the given ConditionObject uses this synchronizer as its
	 * lock.
	 * 
	 * @param condition
	 *            the condition
	 * @return <tt>true</tt> if owned
	 * @throws NullPointerException
	 *             if the condition is null
	 */
	public final boolean owns(ConditionObject condition) {
		if (condition == null)
			throw new NullPointerException();
		return condition.isOwnedBy(this);
	}

	/**
	 * Queries whether any threads are waiting on the given condition associated
	 * with this synchronizer. Note that because timeouts and interrupts may
	 * occur at any time, a <tt>true</tt> return does not guarantee that a
	 * future <tt>signal</tt> will awaken any threads. This method is designed
	 * primarily for use in monitoring of the system state.
	 * 
	 * @param condition
	 *            the condition
	 * @return <tt>true</tt> if there are any waiting threads
	 * @throws IllegalMonitorStateException
	 *             if exclusive synchronization is not held
	 * @throws IllegalArgumentException
	 *             if the given condition is not associated with this
	 *             synchronizer
	 * @throws NullPointerException
	 *             if the condition is null
	 */
	public final boolean hasWaiters(ConditionObject condition) {
		if (!owns(condition))
			throw new IllegalArgumentException("Not owner");
		return condition.hasWaiters();
	}

	/**
	 * Returns an estimate of the number of threads waiting on the given
	 * condition associated with this synchronizer. Note that because timeouts
	 * and interrupts may occur at any time, the estimate serves only as an
	 * upper bound on the actual number of waiters. This method is designed for
	 * use in monitoring of the system state, not for synchronization control.
	 * 
	 * @param condition
	 *            the condition
	 * @return the estimated number of waiting threads
	 * @throws IllegalMonitorStateException
	 *             if exclusive synchronization is not held
	 * @throws IllegalArgumentException
	 *             if the given condition is not associated with this
	 *             synchronizer
	 * @throws NullPointerException
	 *             if the condition is null
	 */
	public final int getWaitQueueLength(ConditionObject condition) {
		if (!owns(condition))
			throw new IllegalArgumentException("Not owner");
		return condition.getWaitQueueLength();
	}

	/**
	 * Returns a collection containing those threads that may be waiting on the
	 * given condition associated with this synchronizer. Because the actual set
	 * of threads may change dynamically while constructing this result, the
	 * returned collection is only a best-effort estimate. The elements of the
	 * returned collection are in no particular order.
	 * 
	 * @param condition
	 *            the condition
	 * @return the collection of threads
	 * @throws IllegalMonitorStateException
	 *             if exclusive synchronization is not held
	 * @throws IllegalArgumentException
	 *             if the given condition is not associated with this
	 *             synchronizer
	 * @throws NullPointerException
	 *             if the condition is null
	 */
	public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
		if (!owns(condition))
			throw new IllegalArgumentException("Not owner");
		return condition.getWaitingThreads();
	}

	/**
	 * Condition implementation for a {@link AbstractQueuedSynchronizer} serving
	 * as the basis of a {@link Lock} implementation.
	 * 
	 * <p>
	 * Method documentation for this class describes mechanics, not behavioral
	 * specifications from the point of view of Lock and Condition users.
	 * Exported versions of this class will in general need to be accompanied by
	 * documentation describing condition semantics that rely on those of the
	 * associated <tt>AbstractQueuedSynchronizer</tt>.
	 * 
	 * <p>
	 * This class is Serializable, but all fields are transient, so deserialized
	 * conditions have no waiters.
	 */
	public class ConditionObject implements Condition, java.io.Serializable {
		private static final long serialVersionUID = 1173984872572414699L;
		/** First node of condition queue. */
		private transient Node firstWaiter;
		/** Last node of condition queue. */
		private transient Node lastWaiter;

		/**
		 * Creates a new <tt>ConditionObject</tt> instance.
		 */
		public ConditionObject() {
		}

		// Internal methods

		/**
		 * Adds a new waiter to wait queue.
		 * 
		 * @return its new wait node
		 */
		private Node addConditionWaiter() {
			Node t = lastWaiter;
			// If lastWaiter is cancelled, clean out.
			if (t != null && t.waitStatus != Node.CONDITION) {
				unlinkCancelledWaiters();
				t = lastWaiter;
			}
			Node node = new Node(Thread.currentThread(), Node.CONDITION);
			if (t == null)
				firstWaiter = node;
			else
				t.nextWaiter = node;
			lastWaiter = node;
			return node;
		}

		/**
		 * Removes and transfers nodes until hit non-cancelled one or null.
		 * Split out from signal in part to encourage compilers to inline the
		 * case of no waiters.
		 * 
		 * @param first
		 *            (non-null) the first node on condition queue
		 */
		private void doSignal(Node first) {
			do {
				if ((firstWaiter = first.nextWaiter) == null)
					lastWaiter = null;
				first.nextWaiter = null;
			} while (!transferForSignal(first) && (first = firstWaiter) != null);
		}

		/**
		 * Removes and transfers all nodes.
		 * 
		 * @param first
		 *            (non-null) the first node on condition queue
		 */
		private void doSignalAll(Node first) {
			lastWaiter = firstWaiter = null;
			do {
				Node next = first.nextWaiter;
				first.nextWaiter = null;
				transferForSignal(first);
				first = next;
			} while (first != null);
		}

		/**
		 * Unlinks cancelled waiter nodes from condition queue. Called only
		 * while holding lock. This is called when cancellation occurred during
		 * condition wait, and upon insertion of a new waiter when lastWaiter is
		 * seen to have been cancelled. This method is needed to avoid garbage
		 * retention in the absence of signals. So even though it may require a
		 * full traversal, it comes into play only when timeouts or
		 * cancellations occur in the absence of signals. It traverses all nodes
		 * rather than stopping at a particular target to unlink all pointers to
		 * garbage nodes without requiring many re-traversals during
		 * cancellation storms.
		 */
		private void unlinkCancelledWaiters() {
			Node t = firstWaiter;
			Node trail = null;
			while (t != null) {
				Node next = t.nextWaiter;
				if (t.waitStatus != Node.CONDITION) {
					t.nextWaiter = null;
					if (trail == null)
						firstWaiter = next;
					else
						trail.nextWaiter = next;
					if (next == null)
						lastWaiter = trail;
				} else
					trail = t;
				t = next;
			}
		}

		// public methods

		/**
		 * Moves the longest-waiting thread, if one exists, from the wait queue
		 * for this condition to the wait queue for the owning lock.
		 * 
		 * @throws IllegalMonitorStateException
		 *             if {@link #isHeldExclusively} returns {@code false}
		 */
		public final void signal() {
			if (!isHeldExclusively())
				throw new IllegalMonitorStateException();
			Node first = firstWaiter;
			if (first != null)
				doSignal(first);
		}

		/**
		 * Moves all threads from the wait queue for this condition to the wait
		 * queue for the owning lock.
		 * 
		 * @throws IllegalMonitorStateException
		 *             if {@link #isHeldExclusively} returns {@code false}
		 */
		public final void signalAll() {
			if (!isHeldExclusively())
				throw new IllegalMonitorStateException();
			Node first = firstWaiter;
			if (first != null)
				doSignalAll(first);
		}

		/**
		 * Implements uninterruptible condition wait.
		 * <ol>
		 * <li>Save lock state returned by {@link #getState}.
		 * <li>Invoke {@link #release} with saved state as argument, throwing
		 * IllegalMonitorStateException if it fails.
		 * <li>Block until signalled.
		 * <li>Reacquire by invoking specialized version of {@link #acquire}
		 * with saved state as argument.
		 * </ol>
		 */
		public final void awaitUninterruptibly() {
			Node node = addConditionWaiter();
			int savedState = fullyRelease(node);
			boolean interrupted = false;
			while (!isOnSyncQueue(node)) {
				LockSupport.park(this);
				if (Thread.interrupted())
					interrupted = true;
			}
			if (acquireQueued(node, savedState) || interrupted)
				selfInterrupt();
		}

		/*
		 * For interruptible waits, we need to track whether to throw
		 * InterruptedException, if interrupted while blocked on condition,
		 * versus reinterrupt current thread, if interrupted while blocked
		 * waiting to re-acquire.
		 */

		/** Mode meaning to reinterrupt on exit from wait */
		private static final int REINTERRUPT = 1;
		/** Mode meaning to throw InterruptedException on exit from wait */
		private static final int THROW_IE = -1;

		/**
		 * Checks for interrupt, returning THROW_IE if interrupted before
		 * signalled, REINTERRUPT if after signalled, or 0 if not interrupted.
		 */
		private int checkInterruptWhileWaiting(Node node) {
			return Thread.interrupted() ? (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) : 0;
		}

		/**
		 * Throws InterruptedException, reinterrupts current thread, or does
		 * nothing, depending on mode.
		 */
		private void reportInterruptAfterWait(int interruptMode) throws InterruptedException {
			if (interruptMode == THROW_IE)
				throw new InterruptedException();
			else if (interruptMode == REINTERRUPT)
				selfInterrupt();
		}

		/**
		 * Implements interruptible condition wait.
		 * <ol>
		 * <li>If current thread is interrupted, throw InterruptedException.
		 * <li>Save lock state returned by {@link #getState}.
		 * <li>Invoke {@link #release} with saved state as argument, throwing
		 * IllegalMonitorStateException if it fails.
		 * <li>Block until signalled or interrupted.
		 * <li>Reacquire by invoking specialized version of {@link #acquire}
		 * with saved state as argument.
		 * <li>If interrupted while blocked in step 4, throw
		 * InterruptedException.
		 * </ol>
		 */
		public final void await() throws InterruptedException {
			if (Thread.interrupted())
				throw new InterruptedException();
			Node node = addConditionWaiter();
			int savedState = fullyRelease(node);
			int interruptMode = 0;
			while (!isOnSyncQueue(node)) {
				LockSupport.park(this);
				if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
					break;
			}
			if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
				interruptMode = REINTERRUPT;
			if (node.nextWaiter != null) // clean up if cancelled
				unlinkCancelledWaiters();
			if (interruptMode != 0)
				reportInterruptAfterWait(interruptMode);
		}

		/**
		 * Implements timed condition wait.
		 * <ol>
		 * <li>If current thread is interrupted, throw InterruptedException.
		 * <li>Save lock state returned by {@link #getState}.
		 * <li>Invoke {@link #release} with saved state as argument, throwing
		 * IllegalMonitorStateException if it fails.
		 * <li>Block until signalled, interrupted, or timed out.
		 * <li>Reacquire by invoking specialized version of {@link #acquire}
		 * with saved state as argument.
		 * <li>If interrupted while blocked in step 4, throw
		 * InterruptedException.
		 * </ol>
		 */
		public final long awaitNanos(long nanosTimeout) throws InterruptedException {
			if (Thread.interrupted())
				throw new InterruptedException();
			Node node = addConditionWaiter();
			int savedState = fullyRelease(node);
			long lastTime = System.nanoTime();
			int interruptMode = 0;
			while (!isOnSyncQueue(node)) {
				if (nanosTimeout <= 0L) {
					transferAfterCancelledWait(node);
					break;
				}
				LockSupport.parkNanos(this, nanosTimeout);
				if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
					break;

				long now = System.nanoTime();
				nanosTimeout -= now - lastTime;
				lastTime = now;
			}
			if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
				interruptMode = REINTERRUPT;
			if (node.nextWaiter != null)
				unlinkCancelledWaiters();
			if (interruptMode != 0)
				reportInterruptAfterWait(interruptMode);
			return nanosTimeout - (System.nanoTime() - lastTime);
		}

		/**
		 * Implements absolute timed condition wait.
		 * <ol>
		 * <li>If current thread is interrupted, throw InterruptedException.
		 * <li>Save lock state returned by {@link #getState}.
		 * <li>Invoke {@link #release} with saved state as argument, throwing
		 * IllegalMonitorStateException if it fails.
		 * <li>Block until signalled, interrupted, or timed out.
		 * <li>Reacquire by invoking specialized version of {@link #acquire}
		 * with saved state as argument.
		 * <li>If interrupted while blocked in step 4, throw
		 * InterruptedException.
		 * <li>If timed out while blocked in step 4, return false, else true.
		 * </ol>
		 */
		public final boolean awaitUntil(Date deadline) throws InterruptedException {
			if (deadline == null)
				throw new NullPointerException();
			long abstime = deadline.getTime();
			if (Thread.interrupted())
				throw new InterruptedException();
			Node node = addConditionWaiter();
			int savedState = fullyRelease(node);
			boolean timedout = false;
			int interruptMode = 0;
			while (!isOnSyncQueue(node)) {
				if (System.currentTimeMillis() > abstime) {
					timedout = transferAfterCancelledWait(node);
					break;
				}
				LockSupport.parkUntil(this, abstime);
				if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
					break;
			}
			if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
				interruptMode = REINTERRUPT;
			if (node.nextWaiter != null)
				unlinkCancelledWaiters();
			if (interruptMode != 0)
				reportInterruptAfterWait(interruptMode);
			return !timedout;
		}

		/**
		 * Implements timed condition wait.
		 * <ol>
		 * <li>If current thread is interrupted, throw InterruptedException.
		 * <li>Save lock state returned by {@link #getState}.
		 * <li>Invoke {@link #release} with saved state as argument, throwing
		 * IllegalMonitorStateException if it fails.
		 * <li>Block until signalled, interrupted, or timed out.
		 * <li>Reacquire by invoking specialized version of {@link #acquire}
		 * with saved state as argument.
		 * <li>If interrupted while blocked in step 4, throw
		 * InterruptedException.
		 * <li>If timed out while blocked in step 4, return false, else true.
		 * </ol>
		 */
		public final boolean await(long time, TimeUnit unit) throws InterruptedException {
			if (unit == null)
				throw new NullPointerException();
			long nanosTimeout = unit.toNanos(time);
			if (Thread.interrupted())
				throw new InterruptedException();
			Node node = addConditionWaiter();
			int savedState = fullyRelease(node);
			long lastTime = System.nanoTime();
			boolean timedout = false;
			int interruptMode = 0;
			while (!isOnSyncQueue(node)) {
				if (nanosTimeout <= 0L) {
					timedout = transferAfterCancelledWait(node);
					break;
				}
				if (nanosTimeout >= spinForTimeoutThreshold)
					LockSupport.parkNanos(this, nanosTimeout);
				if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
					break;
				long now = System.nanoTime();
				nanosTimeout -= now - lastTime;
				lastTime = now;
			}
			if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
				interruptMode = REINTERRUPT;
			if (node.nextWaiter != null)
				unlinkCancelledWaiters();
			if (interruptMode != 0)
				reportInterruptAfterWait(interruptMode);
			return !timedout;
		}

		// support for instrumentation

		/**
		 * Returns true if this condition was created by the given
		 * synchronization object.
		 * 
		 * @return {@code true} if owned
		 */
		final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
			return sync == AbstractQueuedSynchronizer.this;
		}

		/**
		 * Queries whether any threads are waiting on this condition. Implements
		 * {@link AbstractQueuedSynchronizer#hasWaiters}.
		 * 
		 * @return {@code true} if there are any waiting threads
		 * @throws IllegalMonitorStateException
		 *             if {@link #isHeldExclusively} returns {@code false}
		 */
		protected final boolean hasWaiters() {
			if (!isHeldExclusively())
				throw new IllegalMonitorStateException();
			for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
				if (w.waitStatus == Node.CONDITION)
					return true;
			}
			return false;
		}

		/**
		 * Returns an estimate of the number of threads waiting on this
		 * condition. Implements
		 * {@link AbstractQueuedSynchronizer#getWaitQueueLength}.
		 * 
		 * @return the estimated number of waiting threads
		 * @throws IllegalMonitorStateException
		 *             if {@link #isHeldExclusively} returns {@code false}
		 */
		protected final int getWaitQueueLength() {
			if (!isHeldExclusively())
				throw new IllegalMonitorStateException();
			int n = 0;
			for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
				if (w.waitStatus == Node.CONDITION)
					++n;
			}
			return n;
		}

		/**
		 * Returns a collection containing those threads that may be waiting on
		 * this Condition. Implements
		 * {@link AbstractQueuedSynchronizer#getWaitingThreads}.
		 * 
		 * @return the collection of threads
		 * @throws IllegalMonitorStateException
		 *             if {@link #isHeldExclusively} returns {@code false}
		 */
		protected final Collection<Thread> getWaitingThreads() {
			if (!isHeldExclusively())
				throw new IllegalMonitorStateException();
			ArrayList<Thread> list = new ArrayList<Thread>();
			for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
				if (w.waitStatus == Node.CONDITION) {
					Thread t = w.thread;
					if (t != null)
						list.add(t);
				}
			}
			return list;
		}
	}

	/**
	 * Setup to support compareAndSet. We need to natively implement this here:
	 * For the sake of permitting future enhancements, we cannot explicitly
	 * subclass AtomicInteger, which would be efficient and useful otherwise.
	 * So, as the lesser of evils, we natively implement using hotspot
	 * intrinsics API. And while we are at it, we do the same for other CASable
	 * fields (which could otherwise be done with atomic field updaters).
	 */
	private static final Unsafe unsafe = Unsafe.getUnsafe();
	private static final long stateOffset;
	private static final long headOffset;
	private static final long tailOffset;
	private static final long waitStatusOffset;
	private static final long nextOffset;

	static {
		try {
			stateOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("state"));
			headOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("head"));
			tailOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
			waitStatusOffset = unsafe.objectFieldOffset(Node.class.getDeclaredField("waitStatus"));
			nextOffset = unsafe.objectFieldOffset(Node.class.getDeclaredField("next"));

		} catch (Exception ex) {
			throw new Error(ex);
		}
	}

	/**
	 * CAS head field. Used only by enq.
	 */
	private final boolean compareAndSetHead(Node update) {
		return unsafe.compareAndSwapObject(this, headOffset, null, update);
	}

	/**
	 * CAS tail field. Used only by enq.
	 */
	private final boolean compareAndSetTail(Node expect, Node update) {
		return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
	}

	/**
	 * CAS waitStatus field of a node.
	 */
	private static final boolean compareAndSetWaitStatus(Node node, int expect, int update) {
		return unsafe.compareAndSwapInt(node, waitStatusOffset, expect, update);
	}

	/**
	 * CAS next field of a node.
	 */
	private static final boolean compareAndSetNext(Node node, Node expect, Node update) {
		return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
	}
}
